As long as you remain on the astral and lower mental planes, you will be tense and restless, and you will never find the inner silence needed for your spiritual work. By their very nature, ordinary thoughts and feelings produce such effects; it cannot be otherwise.
You must know the nature of each thing. Just as chemists study the nature and properties of chemical elements, so too must you study the nature and properties of psychic elements.
It is the very nature of interested, selfish thoughts and feelings to trigger tension, agitation and disorder in you.
No matter how hard you try to meditate, you will not succeed until you gain control over the movements of your lower nature.